In the production of hydrocarbons from offshore wells in deep seas, so-called Floating Production, Storage and Offloading (FPSO) ships, or similar concepts, are widely used. Ships of this type often rely on disconnectable turret buoy systems, both for mooring and as a means for connecting to the hydrocarbon risers. In general terms, the known turret buoy systems comprise a turret and a buoyant body, where the buoyant body is configured for connection to a receptacle in the vessel (e.g. the FPSO), and the turret is rotatably arranged inside the buoyant body via bearings (slewing bearings or sliding bearings). The turret has connectors for mooring lines and support hang-off structures for risers. Other variants, employing basically the same principle, are known within the industry.
Thus, when the buoyant body is connected to the vessel and the turret is moored to the seabed, the vessel is allowed to weathervane due to the rotational connection between the buoyant body and the turret. The weathervaning capability is often advantageous, as it significantly expands the operational envelope for the vessel.
Another favourable feature of the turret buoy mooring systems is the ability to quickly disconnect (and re-connect) the turret buoy and the vessel. The buoyant body is provided with buoyancy chambers and provides support to the turret (including the risers and mooring lines) when the turret buoy is disconnected. The buoyancy chambers are dimensioned to match the weight of the risers and mooring lines to ensure buoyancy equilibrium at a required submerged depth.
The prior art includes WO 93/22190, which describes a buoy for use in loading or unloading of a flowable medium, especially oil, comprising an outer buoyancy member arranged to be introduced and secured in a submerged downwardly open receiving space in a floating vessel, and a central member which is rotatably mounted in the outer member and is intended for anchoring to the sea bed and arranged for passage of medium between a transfer line which, in operation, is coupled to the lower end of the central member and a tube system on the vessel. The central member is provided with a lower extension body having an outer peripheral portion abutting on and essentially corresponding to the outer periphery of the adjacent end of the outer buoyancy member, and having a lower portion which is downwardly tapering from the outer peripheral portion. A number of fastening means for fastening of the upper ends of anchoring lines for anchoring of the buoy are fastened at intervals along the periphery of the outer peripheral portion of the extension body, and the extension body comprises at least one buoyancy chamber for buoyancy or ballast material.
The prior art also includes WO 2009/031971, which describes a mooring system for a vessel and a method of mooring a vessel. The mooring system comprises a turret structure; a swivel unit mounted on the turret structure; a bearing assembly for rotatably mounting the turret structure at deck level of the vessel such that the turret structure extends into a moonpool of the vessel and such that the swivel unit is disposed above deck level; a plurality of conduits for fluid communication disposed in the turret structure; a buoy structure, with internal buoyancy compartments, retrievable into the moonpool of the vessel and connectable to the turret structure; a locking assembly for mechanically locking the buoy structure to the turret structure. The turret structure is rotatable as one to align the conduits to corresponding riser valve structures on the buoy structure prior to mechanically locking the buoy structure to the turret structure.
Another buoy device employing basically the same principles as the above mentioned turret buoy, is the Catenary Anchor Leg Mooring (CALM) buoy. The CALM buoy is a floating hull with a rotating head to which vessels can moor. The CALM buoy falls under the category of a Single Point Mooring (SPM) typically with a turntable or turret positioned above the geostationary hull mounted on a bearing (e.g. slewing bearing or sliding bearing). Flexible large bore rubber hoses are used to connect the subsea pipeline to the hull. Similar floating hoses are employed when connecting the buoy to a tanker prior to transferring liquid hydrocarbons. Central to the main bearing is a product swivel which allows fluid to transfer between the geostationary hull and rotating turntable while the moored vessel weathervanes.
The prior art includes WO 2014/180687 A1, which shows a mooring system comprising an external turret (to which a production vessel is moored) from which a buoyant body may be connected and disconnected.
The prior art also includes WO 2011/042535 A1, which discloses a CALM buoy having a floating body anchored to the sea bed via two or more anchor lines. The floating body comprises a lower part to be situated below water level and an upper part to be situated above water level. A turntable is rotatably attached to the floating body upper part via a bearing, the turntable comprising mooring points and fluid transfer means. The floating body is a circular hull, having internal buoyancy chambers.
Although the turret buoy and the CALM buoy to some extent have different technical features and are used for different purposes, they rely on the common principle that one part is moored to the seabed (i.e. more or less geostationary) and another part is connectable to a ship, and the two parts are rotatably connected. For example, in the turret buoy and the CALM turret buoy described above, the geostationary part is the turret and the buoyant body is connected to the ship. By contrast, in a CALM buoy having a turntable, the buoyant body is moored to the seabed, while the turntable is connected to the ship.
Known buoy devices of the kind described above are large, heavy, structures, with heights on the order of 10 to 15 metres and maximum diameters up to 15 metres. As the buoys must be designed with a certain degree of contingency, i.e. in the event one or more of the buoyancy chambers are compromised (e.g. as a result of collision or dropped objects), the known buoys normally have several buoyancy chambers, separated by internal bulkheads. This adds weight and fabrication time. Typical dry weights range between 100 and 1500 Tonnes. Time, equipment, and manpower (and hence cost) required to construct, transport and install known buoy devices are therefore considerable. It is an object of the invention to present a buoy which alleviates at least some of the disadvantages with the prior art.